T. Codi'hujton — On the Chesil Bank. -^ 



VI. — Some Eemarks on tiiio Formation of the Chesil Bank. 

 By Tuomas Codrington, F.G.S. 



THE Paper " On the Formation of tlie Chesil Bank," which appeared 

 in the Geological Magazink for Octoher last, is prefaced by a 

 statement that the authors (Messrs. Bristow and Whitaker) do not 

 propose to enter into the question of the way in which the shinglo 

 was heaped up. But the way in which the shingle was heaped 

 up is intimately connected both with the formation of the Chesil 

 Bank and with its position in advance of the shore line, and, if 

 rightly understood, any speculations on the waste of the coast behind 

 the bank by sub-aerial denudation are superfluous. 



The subject is fully gone into in Mr. Coode's paper " On the Chesil 

 Bank" (Minutes of Proceedings Inst. Civil Engineers, 1853), and 

 in the discussion which follows it, and the action of the sea in 

 throwing up beach is one upon which those who have paid attention 

 to the subject are pretty well agreed. 



The origin of the Chesil and similar shingle banks is due entirely 

 to the heaping-up action of waves breaking Mdien they reach a depth 

 of water about equal to their own height, and when a deep sea wave 

 comes suddenly into shallow water, whether it be close in shore, or 

 over a shoal or bank, shingle will be thrown up, if present. With 

 this action tidal currents and eddies have nothing to do. 



The authors of the paper endeavour to draw a distinction between 

 the Chesil Bank and other shingle banks, but there is no essential 

 difference. The Chesil Bank is undoubtedly the largest and most re- 

 markable, but the Orfordness shingle bank rivals it in length. The 

 latter is separated from the shore for ten miles, and it is a matter of 

 history that it has increased in length five miles in three centuries. 

 The growth of other similar banks has been observed, and a small 

 but instructive example is furnished by the shingle bank now form- 

 ing across the mouth of Christchurch Harbour. 



About twenty years ago the working of ironstone in the Brackle- 

 sham beds of Hengistbury Head was commenced, and some large 

 blocks which had before fallen from the cliff on to the foreshore were 

 removed. The result has been that an accumulation of beach, 

 derived from the gravel capping the cliffs to the westward, which 

 had previously only travelled round the headland in small quantities, 

 was let loose, and a shingle bank a mile in length has already 

 formed, and is now growing at the rate of about forty yards a year, 

 with every prospect of continuing to do so until the accumulation of 

 beach to the westward has been reduced and the balance restored, or 

 until proper measures are taken to stop the travel of the beach 

 round Hengistbury Head. 



At the Chesil Bank all the conditions exist for the formation of an 

 isolated bank. There is unusually deep water close in shore, so that 

 the Atlantic waves come in with full force, and under the shingle 

 there is a bank of clay at about the level of low water. Before Mr. 

 Coode had proved its existence at several points by boring, the Astro- 

 nomer Eoyal had suggested that a bank or a shoal, enough to make 



